Water-soluble vitamin deficiency and toxicity Quiz: Ace Your Exams

A 56-year-old man comes to the clinic with generalized fatigue and dyspnea on exertion that he noticed over the past month. He now becomes breathless after walking up a flight of stairs. He has also noticed numbness in his lower limbs over the last 2 weeks. Past medical history includes chronic pancreatitis. He has been drinking a pint of vodka daily for the past 25 years. Temperature is 37.0°C (98.6°F), pulse is 92/min, respirations are 19/min, and blood pressure is 135/85 mmHg. BMI is 20 kg/m2. He is alert, cooperative, and oriented to time, place, and person. Neurological examination reveals symmetrical peripheral neuropathy in a “stocking and glove” distribution with defective perception of touch and vibration sensation. Motor strength is 3/5 in the lower limbs and 5/5 in the upper limbs bilaterally. Symmetrical muscle wasting is noted in both lower limbs. Deep tendon reflexes are absent at the ankles bilaterally. On cardiac auscultation, a 3rd heart sound is heard, and the apical impulse is displaced to the 6th intercostal space. Romberg sign is negative. Laboratory investigations are shown below:

 

Laboratory value	Result
Complete blood count
Hemoglobin	13 g/dL
Hematocrit	40%
Leukocyte count	5,000/mm3
Platelet count	150,000/mm3
MCV	88 fL

Serum methylmalonic acid levels are normal. This patient’s condition is most likely associated with which of the following impaired biological processes?